Wow, where do you start with Self Service and Portals. So much has changed now, it is difficult to know what is what.
Chatter Answers in Sites
Or Knowledge in Sites
A way of putting all your self service options out on the web
A nice article about Portals http://www.shellblack.com/portals/self-service-portal/
(EE additional cost)
A typical knowledge base. Not customer created. But customers can see it on your public website or on your communities.